Drain Risk Index v1.0: state-level environmental pressure on residential drain and sewer lines in the United States

A state-level index of environmental pressure on residential drain and sewer lines across 37 US states. The index combines three published factors, each normalised to 0-100 and averaged without weighting: frost-line depth, which drives the seasonal ground movement that opens joints in a buried lateral and admits tree roots; municipal water hardness, which deposits scale that narrows the bore over decades; and US Census median year structure built (American Community Survey 5-Year Estimates 2023, Table B25035), used as a proxy for buried pipe material. Principal finding: housing age outweighs water chemistry. Utah and Arizona record the hardest water in the index, both scoring 100 on the hardness sub-score, yet rank 7th and 20th overall because their housing stock is comparatively new (median build years 1996 and 1998). Iowa, with an unremarkable hardness sub-score of 43, ranks 2nd on a median build year of 1950 - the oldest housing stock in the index. Minnesota ranks 1st on an 80-inch frost line. Mississippi ranks last at 17. Coverage: 37 states are ranked. Arkansas is excluded because no water-hardness figure was available; it is omitted rather than estimated. Limitations are documented in full in the accompanying METHODOLOGY.md. In summary: frost depth is a local code determination presented here at state level; water hardness varies by utility rather than by state; median year built is a proxy for pipe material and not an observation of it; and the index describes state-level conditions, not individual properties. Version 1.0 does not incorporate tree canopy, soil type, storm intensity, or utility maintenance history. The composite score is reproducible from the three published sub-scores in the dataset by taking their unweighted mean.